1. Field of the Invention
The present invention relates to an embossing device for embossing a single-ply or multi-ply web of tissue paper.
2. Technical Background
It is generally known that an embossing pattern on a web of tissue paper is applied using one or more steel rolls in combination with rubber rolls. The rubber rolls have a yielding surface which is elastically pressed by a raised embossing pattern on the steel roll. Tissue paper fed between the rubber roll and the steel roll is imparted with an embossing pattern into the sheet.
It is desired to combine a high degree of softness, which contributes to a good feeling for the user, with an appealing aesthetic appearance of the tissue paper. Therefore, a combination of different embossing patterns can be used. An embossing pattern with a regular arrangement of small protuberances in the embossed tissue paper contributes to a voluminous and soft feeling of the embossed paper, whereas the addition of an embossed aesthetic pattern serves to improve the aesthetic appearance.
There is a constant desire to obtain a good visual appearance of the graphic embossing pattern against a background pattern of rather densely distributed protuberances.
U.S. Pat. No. 4,320,162 describes a method for fabricating a multiple fibrous sheet structure in which each web is embossed with a first pattern embossment and a second pattern embossment, having different heights. The individual embossed webs are adhesively joined only in those portions of the embossments with greater height. The provision of greater height to the graphic embossments imparts a better visibility against the background pattern of smaller embossments. In addition, these so-called double-depth embossments are said to enhance the softness, the water retention and the bulk of a two-ply web.
EP 0 797 705 B1 describes an embossed paper sheet having first protuberances and second protuberances which are arranged according to a first and second pattern respectively. The first pattern is a graphic pattern sparingly distributed on the surface of the embossed paper sheet, whereas the second embossing pattern forms the background pattern distributed in a high concentration per surface area of the embossed paper sheet. The graphic embossing pattern is formed by protuberances which have a summit surface of linear form. This choice of distribution and form of the protuberances of the graphic embossing pattern and background pattern serves to improve the aesthetic appearance of the embossed paper sheet. The tissue paper plies are embossed in the nip between a metal edged roll and a rubber roll. The projecting elements on the metal roll forming the graphic and background pattern respectively can differ in height. The projecting elements forming the background pattern are lower than those projecting elements forming the graphic pattern.
All the above cited prior art documents relate to technical solutions which can serve to enhance the visibility of a graphic pattern applied to a tissue paper web in addition to that of a background pattern.
U.S. Pat. No. 5,269,983 relates to a rubber-to-steel mated embossing which uses a resilient roll in which recesses are formed. These laser cut recesses receive protrusions of a rigid male embossing roll when the rolls are placed in contact. Due to the indentations in the rubber roll, significantly less pressure is required between the rubber roll and the rigid roll for causing the rubber to press the web around the protrusions and against the land areas of the rigid roll. This reduces: the wear of the rubber
It is an object of the invention to provide an embossing device which improves the visual appearance of a specific embossing pattern of embossed tissue paper.
It is an underlying principle of the invention to use a pair of rolls for the embossing, namely a steel roll with embossing protuberances on its peripheral surface and a second roll with a yielding layer on its peripheral surface cooperably disposed with respect to the steel roll as to form an embossing nip therewith. The steel roll and the second roll with the yielding layer run in synchronization.
In an embodiment of the present invention, the second roll has discrete zones on its peripheral surface with a different thickness of the yielding layer and/or different characteristics of the yielding layer compared to that of the remaining peripheral surface
The yielding layer of the second roll can be, for example, thicker in those zones which in the embossing nip are opposing the protuberances forming the graphic pattern. This improves the visibility of the graphic pattern and intensifies the graphic appearance even though the height of the protuberances of the graphic pattern and the background pattern are the same. Therefore, the production of the steel embossing roll with an edged metal surface remains relatively simple because the height of the protrusions on the peripheral surface of the metal embossing cylinder are identical to those protuberances forming the background pattern and to those forming the decor pattern.
In a second embodiment of the present invention, the material characteristics of the yielding layer are changed in the discrete zones which, in the embossing nip, are opposed to the embossing protuberances of the steel rolls.
The material characteristics of the yielding layer are mainly responsible for the appearance of the embossing pattern so that the choice of different material characteristics of the yielding layer gives the possibility of influencing the deformation of the paper web in the embossing nip and the resulting aesthetic appearance. Wherein the first mentioned possibility of providing the yielding layer with a greater thickness in the discrete zones influences the outer shape of the second embossing roll, the provision of different material characteristics does not change the outer shape of the second roll. It should be noted that the provision of a bombage of the rolls, which is used to compensate a certain sagging of the rolls, is not to be interpreted as discrete zones with a greater thickness because those zones are not specifically opposed to certain embossing protuberances of the steel counter-roll.
The individual zones can have exactly the same shape as the embossing protuberances of the steel roll. It is, however, also possible to follow only the general shape of the embossing protuberances or to place the zones roughly opposed to the embossing protuberances of the counter roll. For complex geometries of the protuberances, an approximately opposed position of the individual zones might be the only feasible way to achieve the desired results without incurring high costs for the manufacturing of the second roll
According to a preferred embodiment of the invention, the steel roll has first embossing protrusions forming a graphic pattern and second embossing protrusions forming a background pattern wherein the height of the first embossing protrusions and the height of the second embossing protrusions are identical. As outlined above, the provision of protrusions, all having the same height, makes it easier to produce the steel rolls and in turn reduces the overall costs for the embossing device.
In a preferred embodiment, the synchronization of the steel roll and the second roll, preferably a rubber coated roll, places the discrete zones of the rubber coated roll opposing to the first protuberances forming a graphic pattern. During the embossing of a paper web, a graphic pattern is produced which has a better visibility and visual appearance than the second background pattern. However, the inventive device can also be used to accentuate the background pattern over the graphic pattern by it becoming less pronounced and less visible. However, under normal circumstances, an improvement in the aesthetic appearance of the graphic pattern is desired.
According to a preferred embodiment of the invention, the second roller has an additional yielding layer in the discrete zones of its peripheral surface. In other words, there is a first yielding layer, e.g., a rubber layer, covering the whole peripheral surface of the second roll and, on top of it, a second yielding layer covering only the discrete zones.
According to a further preferred embodiment, the additional yielding layer has a different hardness compared to that of the yielding layer. For example, it is possible to provide an additional yielding layer with a lower rubber hardness than that of the yielding layer covering the remaining part of the peripheral surface of the second roll.
The provision of different material characteristics is not only possible in combination with a higher overall thickness of the second roll. It is also possible to produce a second roll which is covered by a yielding layer with discrete zones, the material characteristics of which are different. Alternatively, the second roll can be provided with a yielding layer of uniform thickness which undergoes a surface treatment which is either only applied to the discrete zones or applied to the remaining surface. Thus, the different material characteristics can either be generated by using a different material composition or by using a different surface treatment.